1. Field of the Invention
The present invention relates to a projection lens and more particularly to a projection lens for a high definition video projector which produces on a screen an enlargement of an image appearing on a high definition cathode-ray tube.
2. Description of the Prior Art
Conventional projection lenses are useful for a cathode-ray tube (CRT) on which an image is formed by 525 scanning lines, and most of them do not have a color correction function. There are known some projection lenses having an improved imaging capability attained by color correction. One projection lens constituted by plastic lenses alone is known as disclosed in Japanese Laid-Open Patent Application No. 59-170812 which is constituted by four lens units each composed of four lenses with aspherical surfaces. Further, a projection lens of a hybrid construction (constituted by plastic and glass lenses), is known which is constituted by four lens units each composed of five lenses, as disclosed in Japanese Laid-Open Patent Application No. 58-139111.
However, with these conventional projection lenses having a color correction function and constituted by plastic lenses, a good image quality cannot be ensured because of a large focus drift caused by a change in the ambient temperature. On the other hand, with the hybrid construction lenses which are less influenced by temperature variations, the aperture ratio is small, being in the range of 1:1.3 to 1:1.4, and so it is impossible to obtain a bright projected image. Further, in both constructions, because of large residual aberrations there has been a serious problem of deficiency of performance as a projection lens for use in a video projector of a high definition having 1,125 scanning lines. Besides, because of a small half field angle of about 20.degree. to 23.degree., the projection distance, namely, the distance from a screen to the fore end of the projection lens, must be large in order to obtain a large picture plane. This requirement has been a problem in reducing the size of a projection television.